This invention relates to the field of digital transmission.
In long distance transmission systems, where a typical transmission path may include a considerable number of switches and patch connections, including automatic protection switching to standby facilities, the transmitted signal can be too easily inverted. That is, in telephone parlance, the tip and ring connections can be easily reversed. For many signals such as analog voice signals, reversal is not serious because the signal is totally symmetrical. For other signals, such as television signals, the signal is polarized, but the polarity can be recognized because of asymmetrical characteristics of the signal itself. U.S. Pat. No. 2,820,181, which issued Jan. 14, 1958 to B. M. Bowman et al., describes a circuit for detecting and correcting the polarity of a television signal.
Digital signals, on the other hand, often have symmetrical waveforms for transmission, but their sensitivity to tip-ring reversals depends upon their particular decoding algorithms. Some efficient digital coding schemes such as Class IV (partial response) coding produce a symmetrical voltage waveform for transmission but use an unsymmetrical decoding algorithm. Such coding schemes, therefore, require an uninverted signal for correct decoding, even though the polarity of the signal itself is not recognizable.
An object of this invention is to identify the polarity of a transmitted digital signal, through the addition of a polarity signal.
A second object is to correct the polarity of a received inverted digital signal.
Polarity signals added to a digital signal would normally require extra bandwidth in order to avoid interfering with the digital signal.
A third object of this invention is to add a polarity signal to a digital signal without increasing the transmission bandwidth and without interfering with the digital signal.